MÓDULOS PROFESIONALES a) Denominación y duración

When discussing Si and C terminated surfaces they can also be described as the (0001) and (000¯1) surfaces respectively. While the preparation processes on (0001) surfaces can lead to either Si or C termination, the (000¯1) surface will, within the scope of this thesis, always be C terminated. The reasons for this will become clear throughout this chapter. At this point it would be appropriate to introduce the general concept of surface reconstructions.

As discussed in the introduction to this thesis, when the bulk structure of a crystal is terminated, in this case on a semi-infinite scale where the ratio of surface area to volume is high, a frozen bulk termination of the crystal can be imagined for the surface structure of the terminating face. In this picture of the frozen bulk termination we can imagine an array of dangling bonds across the surface leading to a surface charge density that is unbalanced from the bulk making these surfaces energetically unfavourable. In UHV conditions the frozen bulk structure can almost be realised since very few atoms or molecules are present in the system to saturate the dangling bonds.

Therefore under such conditions surface reconstructions both stable and meta- stable can be observed. Factors such as temperature and terminating crystal plane will affect how a surface reconstructs. In UHV conditions, dangling bonds are ener- getically unfavourable and as a result, with temperature or otherwise, the structure at the surface may change to saturate some or all of these dangling bonds. This is conducted within the selvedge; the top few planes of the crystal proximate the sur- face. It is within this region that changes in surface atomic configuration can result in changes to electronic structure, as described in the case of polar surfaces and in- terfaces in the introduction to this thesis. Therefore it is important to understand the platform that the thin film growth will start from on the substrate surface.

6.2.1

The

√3 ×√3 - R30

reconstruction

The first of the 6h-SiC(0001) surface reconstructions to discuss is the √3 ×√3- R30◦ surface. The naming of the surface comes under wood notation and describes the translation of the bulk in-plane lattice vectors required to form the surface re- construction. In this case the surface lattice spacing is√3 times larger than the bulk lattice spaceing and rotated by 30◦ from the bulk lattice vectors. Figure 6.2 illustrates the √3 ×√3-R30◦ reconstruction and its relationship to the bulk.

Figure 6.2: A model of the√3 ×√3-R30◦ reconstruction over the underlying bulk structure both viewed from above (top) and from the side [120].

The reconstruction described by figure 6.2 shows how Silicon atoms are arranged to reduce the number of dangling bonds on the surface. The model produced by Fujino et al. [120] shows the surface Si atoms tetragonally bonded to subsurface atoms with just one unsaturated bond remaining per surface Si atom. The tetragonal site sits over a first subsurface carbon atom. Another atomic site predicted for this surface was the hexagonal site, positioned over a further subsurface carbon atom however both theoretical calculations and STM experiments confirmed that the tetragonal site was preferred reducing the surface energy of 6h-SiC(0001) [120–122]. The tetragonal and hexagonal sites are termed T4 and H3 respectively. The differences in position of the

two sites on the surface is shown in figure 6.3. This model also supports the features observed by RHEED that will be discussed in chapters 7 and 8.

Figure 6.3: A model showing the atomic position of an Si atom in the T4 site (top) and H3 site (bottom), viewed in cross-section. Si atoms are pink while C atoms are green.

Since there is a single dangling bond left per atom on this surface this surface may still be sensitive to changes in the local chemical potential. An example of this would be a change in atmosphere or the presence of contaminants. When Si is exposed to atmosphere SiOx starts to form. This process also occurs on SiC and can form

a silicate adlayer reconstruction based on the√3 ×√3-R30◦ where an oxygen atom bonds to the remaining dangling bond of this surface reconstruction [123–125]. This adlayer is notoriously difficult to remove from both Si and SiC. Methods of preventing the oxide adlayer formation will be discussed within this chapter.

An alternative means of saturating dangling bonds on surfaces would be to add more material. Adsorption and desorption of material on surfaces is the premise behind catalysis however by working in UHV conditions the species adsorbed can be controlled. An example of this is the oxidation of Si at the surface.

6.2.2

Preparation of 6h-SiC(0001) surfaces by Hydrogen

etching

Hydrogen etching of substrates is a well founded technique for preparing atom- ically flat surfaces. Typically the etching is applied in the form of HF or buffered-HF [126]. One problem with using HF is that it is dangerous to handle and as a result experimentalists have turned to etching with hydrogen gas in a remote environment. Only a small amount of hydrogen gas is required resulting in miniscule amounts of hydrogen gas being exhausted from a system. Most substrate materials have a reason for being etched and for SiC there are two main reasons for treating the surface with hydrogen. The first reason is the removal of silica that can form when SiC surfaces are exposed to air and the second reason is to remove excess carbon from the surface when annealing SiC at high temperatures. The latter becomes necessary when trying to maintain a Si terminated surface at high temperatures. High temperatures are required to remove stubborn hydrocarbons and oxygen atoms. Above 850◦C Si will desorb from SiC but carbon will remain. It is this process that allows graphitic layers to form on the SiC surface and, under certain conditions, establish epitaxial graphene [127]. Substrate preparation methods described in the literature describe how to pro- cess SiC substrates with molecular hydrogen. These methods comprise annealing in separate systems (ex-situ) or annealing within the same system, for example in the same system that is used for MBE growth (in-situ).

A popular experimental setup for preparing SiC substrates by hydrogen is using a tube furnace. Some tube furnace materials vary but the principle is the same. The furnace used for this work comprises an Al2O3 tube heated by an electric heater. Gas

lines can be integrated to provide flow inside the tube either in static confinement or dynamic flow using valves within the furnace. The furnace described is capable of heating the tube up to 1500◦C. Early annealing experiments with hydrogen etching of SiC required heating substrates to over 1500◦C under 1 atm of molecular hydrogen gas pressure. The high temperature causes hydrogen molecules to dissociate and the formed hydrogen atoms bond with C on the SiC surface to form hydrocarbons. De- sorption of hydrocarbons compensates Si desorption at the same elevated temperature [128–131].

This method is not always successful and reports from the literature show that the hydrogen etching can be too aggressive creating rough surfaces with pits and holes on the surface [129, 132]. These pits and holes are considered to be a sign of

incomplete etching in the report by Ramachandran et al. Similar results were obtained when preparing substrates for this work with a tube furnace. Due to limited control of parameters and atmospheric composition within the tube furnace, the decision was made in this work to prepare samples in-situ by preparing the 6h-SiC(0001) surface with hydrogen provided from a hydrogen cracker as described in chapter 2. Improved surface etching has been reported from the use of atomic hydrogen in HV thanks to reduced contamination and improved hydrogen cracking efficiency from using a hydrogen cracker [133, 134].

In the results presented in this thesis the above methods were combined, provid- ing molecular hydrogen to 6h-SiC substrates heated to >800◦C. The total pressure measured by the ion gauge in the UHV system during hydrogen exposure read 5×10−8 mbar. The local pressure at the sample is higher because the sample is in close prox- imity to the hydrogen source and the ion gauge is not calibrated for hydrogen. A correction factor should be applied to the ion gauge reading. Attempts to prepare samples by atomic hydrogen etching using a gas cracker were made however the SiC surface formed was not consistent. The background pressure in the chamber also rose considerably when operating the gas cracker, increasing the risk of contaminating the SiC surface.